Digital servo-control systems generate a control signal in the form of a number which is generally binary. If the equipment that is to be servo-controlled is designed for receiving an analog signal (usually a voltage), then it is necessary to provide a digital-to-analog converter which converts a binary number into a voltage proportional to said number. The resolution of such a system is determined by the resolution of the converter which is itself a function of its capacity measured as a number of bits.
The 12-bit converters that are commercially available are manufactured by numerous manufacturers and they are not expensive. 16-bit converters are also commercially available. The largest capacity converters that are available at present are 18-bit converters, and they are very expensive.
European patent number EP-A-0 043 536 describes a two-module converter in which each module may itself be considered as being a converter. The first module provides coarse adjustment while the second provides fine adjustment with the output analog signal being the sum of the output signals from the two modules. As a result, resolution is limited by the resolution of the second module. In addition, this circuit is designed to be integrated on a single component and it is poorly suited to being made up from standard converters. This solution is therefore expensive.
An object of the present invention is thus to provide a digital servo-control system capable of being embodied using standard components while providing increased resolution at low cost. It makes servo-control of equipment possible at a resolution which is better than that of currently available converters.